[@mantine/form] Allow validation to work with readonly array types

[evantrimboli]

[rtivital]

Please provide an example of the issue that this PR resolves.

[evantrimboli]

See the example here:

https://codesandbox.io/p/sandbox/mantine-react-template-forked-y7wqdp?file=%2Fsrc%2FApp.tsx

Note the type errors for the provided validators.

[rtivital]

Thanks!